This invention relates to video signals and, more particularly, to effective synchronization of video signals.
An image viewed on a television monitor may be transmitted there from a number of sources. Both live broadcasts and taped programming are examples of video signals that may be sent to the television monitor. These video signals are often combined with personal computer (PC) graphics signals. PC graphics, typically created on a processor-based system, may be combined with the television signal prior to viewing on a television display.
A set-top box is a processor-based system that employs a television monitor instead of a computer monitor for viewing video signals, PC graphics signals, or a combination of the two. The set-top box may execute application software, such as electronic mail programs and web browsers, connect to a data network such as the Internet, and receive and display television program signals.
Set-top boxes may combine a broadcast video signal with a graphics signal. The set-top box receives the video signal from an external source, such as via a coaxial cable, and mixes the signal with the PC graphics signal, typically generated from within the set-top box.
Because some processing of the incoming video signal is generally performed in the set-top box, a frame buffer may provide temporary storage of the video signal. Processing operations may include scaling, mixing, color conversion, and filtering, to name a few. These operations are typically performed by a video decoder and/or graphics controller inside the set-top box.
In addition to active video, the incoming video signal includes other information with which the set top box properly decodes the intended image. A horizontal synchronization, or hsync, signal, for example, precedes each scan line of active video. A vertical synchronization, or vsync, signal precedes each field of active video. A color burst signal supplies a reference by which the set top box decodes the color information within the active video portion of the video signal.
Typically, only the active video portion of the video signal is stored in the set-top box frame buffer. To generate the set-top box's output display signal, the horizontal sync, vertical sync, and color burst are regenerated within the set-top box, coupled with the processed active video and PC graphics, and sent to the television monitor. The television monitor thus may display the image as an adaptation of the signal originally received into the set-top box.
The display signal's horizontal sync, vertical sync, and color burst signals are generated using a pixel clock. The pixel clock is typically a high-frequency square wave generated by a phase-locked loop (PLL). The PLL, in turn, may use a crystal oscillator as a frequency reference.
Crystal oscillators are fairly accurate. Nevertheless, crystal oscillators are manufactured with certain tolerances, or inaccuracies, which may affect their performance. The inaccuracies may be particularly evident when subjected to changes in temperature. The inaccuracies reflect through the pixel clock PLL, and, consequently, may affect the timing of other signals recreated during video processing.
The tolerance of a device is usually related to its cost. Thus, a lower-cost oscillator may have a wider tolerance range than a higher-cost oscillator. Set-top boxes tend to be lower-cost processor-based systems, relative to desktop and laptop computers, for example. Thus, a set-top box may employ a crystal oscillator with a relatively wide tolerance.
The set-top box's output display color burst is typically generated by a second PLL referenced to the pixel clock PLL. If the color burst frequency is not as expected by the television monitor, the television display may distort the color or may stop displaying color at all, reverting to black and white images, which may also be distorted.
Where the set-top box's output display raster is referenced to the local crystal oscillator, it will drift relative to the incoming video signal's raster, which is generated from a remote frequency reference. As the output raster changes timing/phase relationships with the incoming video, the displayed image of the incoming active video may exhibit certain anomalies. When scaling incoming video, shearing may occur, in which the top portion of the displayed video is from a different incoming field than is the bottom portion. When not scaling, occasional shearing, dropping, or duplication of fields of the incoming video image may occur within the display raster.
Double buffering techniques, in which two or more frames of incoming video are stored in the frame buffer, can overcome the shearing problem, but still result in periodic field duplication or dropping, and take twice as much frame buffer memory as storing a single frame. In addition, this technique delays the video image longer than does single buffering, and can introduce synchronization anomalies with the audio content of the television program, as perceived by the person observing the program.
Hard sync-lock techniques, in which the display raster locks directly onto the capture raster's sync signals, can also overcome the shearing problem if the polarity of the display field is opposite to the polarity of the capture field. However, this results in disturbance of the PC graphics display when the incoming video source changes, for example when changing TV channels. In addition, this technique is not available in many PC graphics hardware chips.
Thus, there is a continuing need to adjust the pixel clock phase-locked loop and the color burst signal to avert display anomalies.